Power amplifier saturation detection

ABSTRACT

In a portable radio transceiver, a power amplifier system includes a saturation detector that detects power amplifier saturation in response to duty cycle of the amplifier transistor collector voltage waveform. The saturation detection output signal can be used by a power control circuit to back off or reduce the amplification level of the power amplifier to avoid power amplifier control loop saturation.

CROSS-REFERENCED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/215,386, filed Mar. 17, 2014 and entitled “POWER AMPLIFIER SATURATIONDETECTION,” which is itself a continuation of U.S. patent applicationSer. No. 13/348,546, filed Jan. 11, 2012 and entitled “POWER AMPLIFIERSATURATION DETECTION,” which is itself a continuation of U.S. patentapplication Ser. No. 12/259,645, filed Oct. 28, 2008 and entitled “POWERAMPLIFIER SATURATION DETECTION.” The entire disclosure of each of theseapplications is hereby incorporated by reference in their entirety.

BACKGROUND

Radio frequency (RF) transmitters of the type used in mobile wirelesstelephones (also known as cellular telephones) and other portable radiotransceivers commonly include transmit power control circuitry thatadjusts the power of the transmitted RF signal. The power controlcircuitry can adjust a power amplifier to increase or decrease thetransmitted RF power. Adjusting transmitted RF power is useful forseveral purposes. For example, in many types of cellulartelecommunications systems, it is useful for transmitted RF power to behigher when the transceiver (also referred to as a handset) is fartherfrom the nearest base station and lower when the transceiver is closerto the nearest base station. Also, in some types of multi-mode (e.g.,dual-mode) transceivers, such as those that are capable of operating inaccordance with both the GSM (Global System for Mobiletelecommunication) standard and EDGE (Enhanced Data rates for GSMEvolution) standard, requirements for transmitted RF power differdepending on whether the transceiver is operating in GSM mode or EDGEmode. Similarly, requirements for transmitted RF power can differ inmulti-band (e.g., dual-band) transceivers, such as those that arecapable of operating in both a GSM “low band” frequency band (e.g., the880-915 MHz frequency band that is used in much of Europe, Africa, theMiddle East and Asia) and a GSM “high band” frequency band (e.g., the1850-1910 MHz frequency band that is used in the United States). Toaccommodate different power amplification requirements for multiplebands, the power amplifier system of the transceiver may correspondinglyinclude multiple power amplifiers.

In some applications, the power amplifier system of a portable radiotransceiver includes a negative feedback power control loop to adjustthe output power of the power amplifier to a level within the tolerancerange specified by the mode under which the transceiver is operating.For example, while a transceiver is transmitting in GSM mode, the powercontrol loop strives to maintain the amplifier output power within thetolerance range specified by the GSM standard for the frequency-shiftkeying-modulated (specifically, Gaussian Minimum Shift Keying (GMSK))signal that is transmitted in accordance with the GSM standard.Likewise, while the transceiver is transmitting in EDGE mode, thecontrol loop strives to maintain the amplifier output power within thetolerance range specified by the EDGE standard for the 8-phase-shiftkeying (8PSK)-modulated signal that is transmitted in accordance withthe EDGE standard. In general, the feedback loop compares a feedbackquantity, such as detected RF output power level, with a referencecontrol voltage. The difference between the two voltages (also referredto as difference error) is integrated and applied to the power controlport of the power amplifier. For GMSK, the power amplifier power controlport is typically a voltage controlled input (V_PC), which adjusts thepower amplifier bias. The RF input level is fixed. For EDGE, the poweramplifier power control port is the RF input level. In EDGE, V_PC canalso be adjusted to optimize efficiency while maintaining linearity. Thelarge loop gain minimizes the difference error and drives the outputpower accuracy to the precision of the loop feedback circuitry andreference control voltage.

A power amplifier control loop can undesirably voltage-saturate underconditions such as insufficient battery power and VSWR (voltage standingwave ratio) load line extremes. Such conditions can cause an undesirabledecrease in control loop gain, increase in difference error, or both.These effects can manifest themselves in sluggish control loop response,resulting in drifting power amplifier output power level or evencomplete loss of control loop lock.

Power control loop saturation can also result in switching spectrumdegradation and nonconformance with applicable transmission standards(e.g., GMSK), such as exceeding power-versus-time (PvT) measurementsspecified by the applicable standard. Furthermore, peaks of anamplitude-modulated EDGE signal envelope can become clipped, causingmodulation spectrum degradation.

To avoid power control loop saturation, some power amplifier systemshave included circuitry that monitors the loop error voltage and reducesthe loop reference voltage until the loop error is eliminated.Alternatively, a power amplifier system can include saturation detectioncircuitry that detects when the control loop is nearing saturation andactivates a “saturation detect” signal. The power control circuitryresponds to this signal by reducing the target output power until thesaturation detection circuitry deactivates the “saturation detect”signal, indicating normal or non-saturated control loop operation.

For example, as illustrated in FIG. 1, the gain, or amplification, of apower amplifier 10 is controlled by a voltage regulator 12 comprising anoperational amplifier 13, a PFET (p-channel field-effect transistor) 14,and associated resistors 16 and 18. Power amplifier 10 can include anumber of cascaded stages, but only the transistor 20 of the final stageis shown for purposes of clarity (other such stages being indicated bythe ellipsis (“ . . . ”) symbol). Voltage regulator 12 is responsive toa power control signal (V_PC) that is produced by power controlcircuitry (not shown for purposes of clarity). Note that the output ofoperational amplifier 13 is coupled via PFET 14 to the collectorterminal of transistor 20. Such an arrangement provides what is known ascollector voltage amplifier control (COVAC).

The circuitry for generating a “saturation detect” signal includes acomparator 22, a current source 24, and a resistor 26. A power supplyvoltage (V_BATT) provided by a battery-operated power supply (not shownfor purposes of clarity) is coupled to the source terminal of PFET 14and one terminal of resistor 26. A power supply-dependent referencevoltage is applied to one terminal of comparator 22 via resistor 26 andcurrent source 24. The other terminal of comparator 22 receives thedrain voltage of PFET 14. If the PFET 14 drain voltage exceeds thecomparator reference voltage, comparator 22 generates a “saturationdetect” signal indicating that the voltage regulator is saturated. Theregulator gain-bandwidth is insufficient to accurately follow the V_PCinput signal, resulting in power amplifier PvT time mask and switchingspectrum specification violations.

While the technique described above with reference to FIG. 1 fordetecting when a power amplifier control loop is in or near saturationis useful in a power amplifier system having a COVAC transistorarrangement, the technique cannot be used in some other cases. Forexample, in some power amplifier transistor arrangements, the collectorof the final-stage transistor is directly connected to the power supplyvoltage (V_BATT).

SUMMARY

Embodiments of the invention relate to a power amplifier system in aportable radio frequency (RF) transmitter or transceiver, to a mobilewireless telecommunication device having such a transceiver, and to amethod of operation of the power amplifier system, where the poweramplifier system includes a saturation detector that detects poweramplifier saturation.

In an exemplary embodiment, the power amplifier circuit includes a poweramplifier, a duty cycle detector, and a comparator section. The poweramplifier has at least one output transistor having an output transistorterminal coupled to a supply voltage. The duty cycle detector canprovide an indication of power amplifier saturation by detecting theduty cycle or ratio between the amount of time that the waveformproduced at the output transistor terminal is negative and the amount oftime that the waveform is positive.

In an exemplary embodiment, the duty cycle detector can include alimiter section and an averaging filter section. The limiter section iscoupled to the output transistor terminal and blocks positive voltageexcursions while passing negative voltage excursions. The averagingfilter section is coupled to an output of the limiter section. Thecomparator section produces a saturation detection output signal bycomparing the signal that is output by the averaging filter section witha reference voltage. The saturation detection output signal can be usedby a power control circuit to back off or reduce the amplification levelof the power amplifier to avoid operating in saturation. The limitersection, averaging filter section, and comparator section or portionsthereof can be embodied in any suitable circuitry or systems, such asdiscrete circuitry formed in an integrated circuit chip, in programmedor configured digital signal processing logic, or in any other suitablecircuitry or systems.

Other systems, methods, features, and advantages of the invention willbe or become apparent to one with skill in the art upon examination ofthe following figures and detailed description. It is intended that allsuch additional systems, methods, features, and advantages be includedwithin this description, be within the scope of the invention, and beprotected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

The invention can be better understood with reference to the followingfigures. The components within the figures are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof the invention. Moreover, in the figures, like reference numeralsdesignate corresponding parts throughout the different views.

FIG. 1 is a schematic diagram of a portion of a prior power amplifiersystem having saturation detection circuitry.

FIG. 2 is a block diagram of a mobile wireless telephone, in accordancewith an exemplary embodiment of the present invention.

FIG. 3 is a block diagram of the transmitter portion of the mobilewireless telephone shown in FIG. 2.

FIG. 4 is a block diagram of the power amplifier system shown in FIG. 3.

FIG. 5 is a flow diagram illustrating a method of operation of the poweramplifier system of FIG. 4.

DETAILED DESCRIPTION

As illustrated in FIG. 2, in accordance with an exemplary embodiment ofthe invention, a mobile wireless telecommunication device, such as acellular telephone, includes a radio frequency (RF) subsystem 30, anantenna 32, a baseband subsystem 34, and a user interface section 36.The RF subsystem 30 includes a transmitter portion 38 and a receiverportion 40. User interface section 36 includes a microphone 42, aspeaker 44, a display 46, and a keyboard 48, all coupled to basebandsubsystem 34. The output of transmitter portion 38 and the input ofreceiver portion 40 are coupled to antenna 32 via a front-end module(FEM) 50 that allows simultaneous passage of both the transmitted RFsignal produced by transmitter portion 38 and the received RF signalthat is provided to receiver portion 40. But for transmitter portion 38,the above-listed elements can be of the types conventionally included insuch mobile wireless telecommunication devices. As conventionalelements, they are well understood by persons of ordinary skill in theart to which the present invention relates and, accordingly, notdescribed in further detail in this patent specification (“herein”).However, unlike conventional transmitter portions of such mobilewireless telecommunication devices, transmitter portion 38 embodiespower amplifier saturation detection features and methods, described infurther detail below. It should be noted that while the invention isdescribed in the context of an exemplary embodiment relating to a mobilewireless telephone, the invention alternatively can be embodied in otherdevices that include mobile or portable RF transmitters.

As illustrated in FIG. 3, a modulator 52 in transmitter portion 38receives the signal that is input to transmitter portion 38. Modulator52 modulates the input signal and provides the modulated signal to anupconverter 54. Upconverter 54 shifts or upconverts the frequency of themodulated signal from a baseband frequency to a transmit frequency andprovides the upconverted signal to a power amplifier system 56. Althoughnot shown in FIG. 2 or 3 for purposes of clarity, power amplifier system56 can also receive one or more control signals from a systemcontroller, which can be included in baseband subsystem 34 or othersuitable element. Such control signals typically relate to adjustingamplifier gain, bias, and other amplifier parameters.

As illustrated in FIG. 4, power amplifier system 56 is based upon ahigh-band power amplifier 60 and a low-band power amplifier 62. Althoughthe invention is described with regard to an exemplary embodiment inwhich the transmitter is of the dual-band type, capable of transmittingin a selected one of two frequency bands (referred to herein as highband and low band), the invention is applicable to power amplifiersystems having as few as a single band and corresponding single poweramplifier. Power amplifiers 60 and 62 can be of a conventional type andcan include a number of cascaded stages, but only the transistor 64 ofthe final stage of power amplifier 60 and the transistor 66 of the finalstage of power amplifier 62 are shown for purposes of clarity (othersuch stages and biasing circuitry being indicated by the ellipsis (“ . .. ”) symbol). High-band power amplifier 60 receives an RF signal 68 tobe amplified, and low-band power amplifier receives an RF signal 70 tobe amplified. Note that power amplifiers 60 and 62 are not of the COVACtype; rather, the collector terminals of transistors 64 and 66 arecoupled directly to the power supply voltage (V_BATT) via inductances 72and 74, respectively. (As used herein, the term “coupled” meansconnected via zero or more intermediate elements.) The power supplyvoltage can be that which is provided by a suitable battery-based powersupply circuit (not shown for purposes of clarity) of the type commonlyincluded in mobile wireless telecommunication devices.

Power amplifier system 56 can further include a power amplifier systemcontroller 76 that provides power control signals 78 and 80 to poweramplifiers 60 and 62, respectively. Power amplifier system controller 76can operate in response to power control signals 82 that it receivesfrom a centralized device controller (not shown) in baseband subsystem34 (FIG. 2) or other suitable portion of the mobile wirelesstelecommunication device. Power amplifier system controller 76 canfurther operate in response to feedback signals 84 and 86 that arerespectively representative of the transmitted high-band and low-band RFsignal power. As such a feedback control loop is conventional in mobilewireless telecommunication devices, it is well understood by persons ofordinary skill in the art and, accordingly, not described in furtherdetail herein.

When a transistor 64 or 66 is not operating in its saturation region,its collector voltage waveform is sinusoidal. It has been found inaccordance with the present invention that as a transistor 64 or 66enters the bipolar device saturation region of its operation, thenegative cycle portion of its collector voltage waveform becomesincreasingly deformed from a sinusoidal shape. That is, entry into thesaturation region affects the negative cycle portion more than thepositive cycle portion. As transistor operation moves deeper and deeperinto the saturation region, the positive cycle portion remainssubstantially sinusoidal, but the negative cycle portion becomesincreasingly square and increases in duty cycle. Accordingly, a valuethat represents the ratio between the amount of time the collectorvoltage waveform is negative and the amount of time the collectorvoltage waveform is positive, i.e., the duty cycle, can provide anindication of saturation depth. Similarly, it can be noted that a valuethat represents the approximate average or mean voltage of the negativecycle portion can also provide an indication of saturation depth. In theexemplary embodiment of the invention, the value is determined asdescribed below. It should be noted that although the term “average” or“averaging” is used herein for convenience, the term is not limited tothe mathematical average (or mean) or a mathematical process, andencompasses within its scope of meaning all quantities that approximateor correspond to such an average or mean, as illustrated by theoperation of the exemplary averaging circuitry described below.

A first limiter circuit 87 that is coupled to the output of high-bandpower amplifier 60 includes a first diode 88. A first averaging filter90 that is coupled to the output of first limiter circuit 87 includes acapacitor 92 and two resistors 94 and 96. Biasing resistors 98 and 100and the voltage provided by a voltage regulator 102 bias diode 88 anddefine the quiescent operating point of diode 88 to be substantially atthe knee voltage of diode 88. In this manner, diode 88 turns on orconducts in response to even small positive voltage excursions or cycleportions of the RF signal at the output of power amplifier 60. Whenconducting, diode 88 clips the positive voltage excursion or cycleportion of the signal at a value of about one diode drop (0.7 V). Diode88 is turned off or does not conduct in response to negative voltageexcursions or cycle portions of the signal. Thus, first limiter circuit87 passes the negative cycle portion and blocks or clips the positivecycle portion. A filter capacitor 104 inhibits the RF signal frominterfering with the operation of other circuitry.

First averaging filter 90 receives the RF signal negative cycle portionthat first limiter circuit 87 passed and low-pass filters or averagesit. The output of first averaging filter 90 thus represents an averageof the negative cycle portion of the RF signal that is output byhigh-band power amplifier 60. Stated another way, the output of firstaveraging filter 90 is representative of the duty cycle, i.e., the ratiobetween the amount of time that the RF signal that is output byhigh-band power amplifier 60 is negative and the amount of time that theRF signal that is output by high-band power amplifier 60 is positive.The combination of first limiter circuit 87 and first averaging filter90 defines a first duty cycle detector.

A second limiter circuit 105 that is coupled to the output of low-bandpower amplifier 62 includes a second diode 106. A second averagingfilter 108 that is coupled to the output of second limiter circuit 105includes a capacitor 110 and two resistors 112 and 114. Biasingresistors 116 and 118 and the voltage provided by voltage regulator 102bias diode 106 and define the quiescent operating point of diode 106 tobe substantially at the knee voltage of diode 106. When conducting,diode 106 clips the positive voltage excursion or cycle portion of thesignal, in the same manner as described above with regard to diode 88.Diode 106 is turned off or does not conduct in response to negativecycle portions. Thus, second limiter circuit 105 passes the negativecycle portion and blocks or clips the positive cycle portion. A filtercapacitor 119 inhibits the RF signal from interfering with the operationof other circuitry.

Second averaging filter 108 receives the RF signal negative cycleportion that second limiter circuit 105 passed and low-pass filters oraverages it. The output of second averaging filter 108 thus representsan average of the negative cycle portion of the RF signal that is outputby low-band power amplifier 62. Stated another way, the output of secondaveraging filter 108 is representative of the duty cycle, i.e., theratio between the amount of time that the RF signal that is output bylow-band power amplifier 62 is negative and the amount of time that theRF signal that is output by low-band power amplifier 62 is positive. Thecombination of second limiter circuit 105 and second averaging filter108 defines a second duty cycle detector.

A comparator circuit includes a comparator 120 and a switching circuitthat comprises two single-pole double-throw switch devices 122 and 124.The pole terminal of the first switch device 122 is connected to a firstinput (e.g., the inverting input) of comparator 120. The pole terminalof the second switch device 124 is connected to a second input (e.g.,the non-inverting input) of comparator 120. The first throw terminal offirst switch device 122 is coupled to the output of first averagingfilter 90 via a resistor 126 and is also connected to a first currentsource 128. The second throw terminal of first switch device 122 iscoupled to the output of second averaging filter 108 via a resistor 130and is also connected to a second current source 132. The first throwterminal of second switch device 124 is similarly coupled to the outputof first averaging filter 90 via resistor 126 and is also connected tofirst current source 128. The second throw terminal of second switchdevice 124 is similarly coupled to the output of second averaging filter108 via resistor 130 and is also connected to second current source 132.Switch devices 122 and 124 and current sources 128 and 132 areresponsive to a band-select signal 134. The state of band-select signal134 indicates either low-band operation or high-band operation. Althoughnot shown for purposes of clarity, other circuitry, which may beincluded, for example, in baseband subsystem 34 (FIG. 2), generatesband-select signal 134 in response to operating conditions in the mannerthat is conventional and well understood in dual-band mobile wirelesstelecommunication devices. It can also be noted that, at any given timewhile the device is transmitting, one of high-band power amplifier 60and low-band power amplifier 62 is active and the other is inactive inaccordance with band-select signal 134. That is, high-band poweramplifier 60 and low-band power amplifier 62 are selectably activatablein response to band-select signal 134.

When band-select signal 134 indicates low-band operation, first switchdevice 122 connects the output of second averaging filter 108 (viaresistor 130) to the first input (e.g., the inverting input) ofcomparator 120, and second switch device 124 connects the output offirst averaging filter 90 (via resistor 126) to the second input (e.g.,the non-inverting input) of comparator 120. (Band-select signal 134 andthe corresponding switch positions are shown in FIG. 4 in the low-bandstate.) In addition, when band-select signal 134 indicates low-bandoperation, current source 128 is active, and current source 132 isinactive. However, as high-band power amplifier 60 is inactive duringlow-band operation, the voltage at the output of first averaging filter90 is constant. This voltage is level-shifted by the effect of resistor126 and current source 128. The level-shifted voltage serves as thecomparator reference voltage and defines the low-band saturationdetection threshold. (Including resistor 126 in the exemplary embodimentprovides a convenient means for selecting or setting the low-bandsaturation detection threshold.)

In low-band operation, as the saturation depth of low-band poweramplifier 62 increases, the voltage at the output of second averagingfilter 108 (which can be referred to as the Vsat_lo signal) decreases.As the decreasing Vsat_lo signal crosses the low-band saturationdetection threshold, comparator 120 produces a high or binary “1” outputsignal, thereby indicating that low-band power amplifier 62 is operatingin (or at least substantially in) saturation. This saturation detectionoutput signal can be provided to power amplifier system controller 76,which responds by adjusting power control signal 80 to indicate areduction in the target amplifier power level. Alternatively, in otherembodiments the saturation detection output signal can be provided toanother element, such as a centralized device controller (not shown) inbaseband subsystem 34 (FIG. 2), which can in turn respond by adjustingpower control signals 82 that power amplifier system controller 76receives. In such an embodiment, power amplifier system controller 76 inturn responds to the adjusted control signals 82 by adjusting powercontrol signal 80 to indicate a reduction in the target amplifier powerlevel.

As low-band power amplifier 62 responds to the change in power controlsignal 80 by reducing the power level of its output RF signal, theVsat_lo signal increases. As the increasing Vsat_lo signal crosses thelow-band saturation detection threshold, comparator 120 toggles toproduce a low or binary “0” output signal, thereby indicating thatlow-band power amplifier 62 is no longer operating in saturation.

When band-select signal 134 indicates high-band operation, first switchdevice 122 connects the output of first averaging filter 90 (viaresistor 126) to the first input (e.g., the inverting input) ofcomparator 120, and second switch device 124 connects the output ofsecond averaging filter 108 (via resistor 130) to the second input(e.g., the non-inverting input) of comparator 120. In addition, whenband-select signal 134 indicates high-band operation, current source 132is active, and current source 128 is inactive. However, as low-bandpower amplifier 62 is inactive during high-band operation, the voltageat the output of second averaging filter 108 is constant. This voltageis level-shifted by the effect of resistor 130 and current source 132.The level-shifted voltage serves as the comparator reference voltage anddefines the high-band saturation detection threshold. (Includingresistor 130 in the exemplary embodiment provides a convenient means forselecting or setting the high-band saturation detection threshold.)

In high-band operation, as the saturation depth of high-band poweramplifier 60 increases, the voltage at the output of first averagingfilter 90 (which can be referred to as the Vsat_hi signal) decreases. Asthe decreasing Vsat_hi signal crosses the high-band saturation detectionthreshold, comparator 120 produces a high or binary “1” output signal,thereby indicating that high-band power amplifier 60 is operating in (orat least substantially in) saturation. This saturation detection outputsignal can be provided to power amplifier system controller 76, whichresponds by adjusting power control signal 78 to indicate a reduction inthe target amplifier power level. As described above with regard tolow-band operation, the saturation detection output signal alternativelycan be provided to a centralized device controller or other element,which can in turn respond by adjusting power control signals 82 thatpower amplifier system controller 76 receives. In such an embodiment,power amplifier system controller 76 in turn responds to the adjustedcontrol signals 82 by adjusting power control signal 78 to indicate areduction in the target amplifier power level.

As high-band power amplifier 60 responds to the change in power controlsignal 78 by reducing the power level of its output RF signal, theVsat_hi signal increases. As the increasing Vsat_hi signal crosses thehigh-band saturation detection threshold, comparator 120 toggles toproduce a low or binary “0” output signal, thereby indicating thathigh-band power amplifier 60 is no longer operating in saturation.

Note that similar variations in reference voltage, diode and resistorvalues between the high-band and low-band circuitry are canceled by thecommon-mode rejection properties of comparator 120.

The above-described elements can be distributed over two or moreintegrated circuit chips 136 and 138 to take advantage of benefits ofdifferent chip process technologies. For example, chip 136 can be formedusing Indium-Gallium-Phosphide (InGaP) Heterojunction Bipolar Transistor(HBT) technologies, and chip 138 can be formed using silicon BiCMOStechnologies that can advantageously integrate bipolar and CMOS devices.

The operation of the above-described power amplifier system 56 ispresented in flow diagram form in FIG. 5. As indicated by block 140,depending on whether the transmitter is operating in high-band orlow-band mode, i.e., depending on which of high-band power amplifier 60or low-band power amplifier 62 is active, either the high-band RF signalor low-band RF signal is amplified. As indicated by block 142, theamplified signal is limited by blocking positive voltage excursions(i.e., the positive cycle portion of the amplified voltage waveform)while passing negative voltage excursions (i.e., the negative cycleportion of the amplified voltage waveform). As indicated by block 144,the limited signal, representing only the negative cycle portion of thewaveform, is averaged. This average or mean value provides an indicationof saturation depth. As indicated by block 146, this value is comparedwith a threshold value (e.g., a reference voltage). A reference voltagecan be obtained from the output of the inactive one of power amplifiers60 and 62, which can function as part of a reference voltage circuit incombination with a current source. As indicated by block 148, if theaverage value falls below the threshold value, a saturation detectionsignal can be produced. The saturation detection signal can be used bypower control circuitry to back off or reduce the target power level forthe active one of power amplifiers 60 and 62.

While various embodiments of the invention have been described, it willbe apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. For example, although in the illustrated or exemplaryembodiment described above the limiter section, averaging filtersection, and comparator section are shown for purposes of illustrationas embodied in discrete circuitry, persons skilled in the art willappreciate that some or all of such sections and elements thereofalternatively can be embodied in suitably programmed or configureddigital signal processing logic. Accordingly, the invention is not to berestricted except in light of the following claims.

What is claimed is:
 1. A power amplifier system comprising: a firstintegrated circuit chip, the first integrated circuit chip including apower amplifier and a limiter in electrical communication with the poweramplifier, the limiter including a diode that clips a positive voltageexcursion and passes a negative voltage excursion to an averagingfilter; and a second integrated circuit chip including a comparator inelectrical communication with the averaging filter and configured togenerate a saturation detection signal based at least in part on anoutput of the averaging filter, the second integrated circuit chipfurther including a first switch and a second switch, the first switchconfigured to electrically connect the output of the averaging filter toa first input of the comparator and the second switch configured toelectrically connect an output of a second averaging filter to a secondinput of the comparator when a first band-select signal is received, theoutput of the second averaging filter serving as a comparator referencevoltage when the first band-select signal is received.
 2. The poweramplifier system of claim 1 wherein the saturation detection signal isused to reduce the amplification level of the power amplifier.
 3. Thepower amplifier system of claim 1 wherein the second integrated circuitchip further includes a power amplifier controller, the power amplifiercontroller configured to adjust a target power level of the poweramplifier based at least in part on the saturation detection signal. 4.The power amplifier system of claim 1 wherein a collector of the poweramplifier is in electrical communication with an inductor, the inductorin electrical communication with a power supply.
 5. The power amplifiersystem of claim 1 wherein the comparator is in electrical communicationwith the second averaging filter, the second averaging filter configuredto provide a level-shifted voltage signal to the comparator, thelevel-shifted voltage signal serving as the comparator referencevoltage.
 6. The power amplifier system of claim 1 wherein the firstswitch is further configured to electrically connect the output of theaveraging filter to the second input of the comparator and the secondswitch is further configured to electrically connect the output of thesecond averaging filter to the first input of the comparator when asecond band-select signal is received.
 7. The power amplifier system ofclaim 6 wherein the output of the first averaging filter serves as thecomparator reference voltage when the second band-select signal isreceived.
 8. The power amplifier system of claim 1 wherein the positivevoltage excursion is clipped at a value of substantially one diode drop.9. The power amplifier system of claim 1 wherein the averaging filter isa low-pass filter.
 10. The power amplifier system of claim 1 wherein thefirst integrated circuit chip and the second integrated circuit chip areformed using different semiconductor materials.
 11. The power amplifiersystem of claim 1 wherein the first integrated circuit chip is formedusing Indium-Gallium-Phosphide (InGaP) and the second integrated circuitchip is formed using silicon.
 12. A method for detecting power amplifiersaturation, the method comprising: receiving a first band-select signalat a power amplifier system, the power amplifier system including afirst power amplifier and a second power amplifier; responsive to thefirst band-select signal, activating the first power amplifier anddeactivating the second power amplifier; amplifying a first outputsignal of the first power amplifier; blocking a positive voltageexcursion of the first output signal and passing a negative voltageexcursion of the first output signal to obtain a first limited signal;averaging the first limited signal to obtain a first averaged signal;and comparing the first averaged signal to a first reference signal at acomparator to obtain a first saturation detection signal, the firstreference signal based at least in part on a signal from the secondpower amplifier.
 13. The method of claim 12 further comprising:receiving a second band-select signal corresponding to a differentfrequency band than a frequency band corresponding to the firstband-select signal; responsive to the second band-select signal,deactivating the first power amplifier and activating the second poweramplifier; amplifying a second output signal of the second poweramplifier; blocking a positive voltage excursion of the second outputsignal and passing a negative voltage excursion of the second outputsignal to obtain a second limited signal; averaging the second limitedsignal to obtain a second averaged signal; and comparing the secondaveraged signal to a second reference signal at the comparator to obtaina second saturation detection signal.
 14. The method of claim 13 whereinthe second reference signal is based at least in part on a signal fromthe first power amplifier.
 15. The method of claim 12 further comprisingmodifying a target power level of the first power amplifier based atleast in part on the first saturation detection signal.
 16. A wirelessdevice comprising: a baseband subsystem configured to generate aband-select signal; and a power amplifier system including a firstintegrated circuit chip and a second integrated circuit chip, the firstintegrated circuit chip including a power amplifier activated based onthe band-select signal and a limiter in electrical communication withthe power amplifier, the limiter including a diode that clips a positivevoltage excursion and passes a negative voltage excursion to anaveraging filter, the second integrated circuit chip including acomparator in electrical communication with the averaging filter andconfigured to generate a saturation detection signal based at least inpart on an output of the averaging filter, the second integrated circuitchip further including a first switch and a second switch, the firstswitch configured to electrically connect the output of the averagingfilter to a second input of the comparator and the second switchconfigured to electrically connect the output of the second averagingfilter to a first input of the comparator when a band-select signal isreceived, the output of the first averaging filter serving as acomparator reference voltage when the band-select signal is received.17. The wireless device of claim 16 wherein the second integratedcircuit chip further includes a first switch and a second switch, thefirst switch configured to electrically connect the output of theaveraging filter to a first input of the comparator and the secondswitch configured to electrically connect an output of a secondaveraging filter to a second input of the comparator based on theband-select signal.
 18. A power amplifier system comprising: a firstintegrated circuit chip, the first integrated circuit chip including apower amplifier and a limiter in electrical communication with the poweramplifier, the limiter including a diode that clips a positive voltageexcursion and passes a negative voltage excursion to an averagingfilter; and a second integrated circuit chip including a comparator inelectrical communication with the averaging filter and configured togenerate a saturation detection signal based at least in part on anoutput of the averaging filter, the second integrated circuit chipfurther including a first switch and a second switch, the first switchconfigured to electrically connect the output of the averaging filter toa second input of the comparator and the second switch configured toelectrically connect the output of the second averaging filter to afirst input of the comparator when a band-select signal is received, theoutput of the first averaging filter serving as a comparator referencevoltage when the band-select signal is received.
 19. A method fordetecting power amplifier saturation, the method comprising: receiving afirst band-select signal at a power amplifier system, the poweramplifier system including a first power amplifier and a second poweramplifier; responsive to the first band-select signal, activating thefirst power amplifier and deactivating the second power amplifier;amplifying a first output signal of the first power amplifier; blockinga positive voltage excursion of the first output signal and passing anegative voltage excursion of the first output signal to obtain a firstlimited signal; averaging the first limited signal to obtain a firstaveraged signal; comparing the first averaged signal to a firstreference signal at a comparator to obtain a first saturation detectionsignal; and comparing a second averaged signal to a second referencesignal at the comparator to obtain a second saturation detection signal,the second reference signal based at least in part on a signal from thefirst power amplifier.
 20. The method of claim 19 further comprising:receiving a second band-select signal corresponding to a differentfrequency band than a frequency band corresponding to the firstband-select signal; responsive to the second band-select signal,deactivating the first power amplifier and activating the second poweramplifier; amplifying a second output signal of the second poweramplifier; blocking a positive voltage excursion of the second outputsignal and passing a negative voltage excursion of the second outputsignal to obtain a second limited signal; and averaging the secondlimited signal to obtain the second averaged signal.